Process for production of low dielectric ceramic composites

ABSTRACT

High strength fiber reinforced ceramic composites having low dielectric constants stable against high temperatures are made possible by post oxidation of 35-60 volume percent ceramic matrix enveloping 20-60 volume percent ceramic fiber.

This is a divisional of patent application Ser. No. 08/429,307, filedApr. 26, 1995, now abandoned.

FIELD OF INVENTION

This invention relates generally to ceramic composites and moreparticularly to high strength fiber reinforced ceramic composites whichhave low dielectric constants and are capable of withstanding hightemperatures.

DESCRIPTION OF THE BACKGROUND ART

Structural ceramic composite materials are utilized in a wide variety ofhigh temperature and high strength applications for electrical andstructural components. It is desirable in certain applications that acomposite also possess a low dielectric constant. One such applicationinvolves the use of ceramic composites to form containers for use inmicrowave processing of materials. Microwaves are reflected to a lesserextent by materials having low dielectric constants and thus containersmade of such materials allow more efficient microwave processing.

For large ceramic structures, it is advantageous to reinforce ceramicswith fibers to make the ceramic less flaw-sensitive and thus morereliable. The reinforcement fibers act as load bearing constituent,restrain crack propagation in the matrix and when matrix strain isexceeded, give the part added strain capability. Thus, for largemicrowavable containers it is particularly advantageous to develop fiberreinforced ceramic composites which have low dielectric constants.

Various composite materials have been disclosed in the prior art whichclaim to provide high strength and high temperature stability. Amongthese materials are carbon/carbon composites, silicon carbide/siliconcarbide composites as well as carbon fiber reinforced silicon carbide,and silicon carbide fiber reinforced carbon composites. However, none ofthese materials can provide a low dielectric constant material,especially at high temperatures where, for example, silicon carbidebecomes increasingly conductive.

Ceramic composites have been made using dielectric fibers such asalumina fiber and dielectric ceramic matrices such as alumina ormullite, but such ceramic composites must be made using hot pressing orhigh temperature sintering processes.

The hot pressing process is severely limited in the size of the partthat can be made. Large structures would require unduly expensive hotpresses which would be impractical at the extreme manufacturingtemperatures and pressures required.

Low dielectric ceramic composites can be made by the sintering of fiberreinforced composites produced using ceramic powder slurries or sol-geltechniques, but such composites contain large amounts of porosity(15%-30%) which so severely weakens matrix properties that the compositeis not useful for microwave processing.

Ceramic composites have also been produced by the pyrolysis of siliconcontaining polymer composites. However, prior studies of this methodhave resulted in impure matrices which contain sufficient amounts ofcarbon to severely impact the low dielectric nature of such composites.J. R. Strife and J. P. Wesson, "A Study of the Critical FactorsControlling the Synthesis of Ceramic Composites from PreceramicPolymers", Dec. 15, 1990, R-90-917810-5, AD-A232-686.

SUMMARY OF THE INVENTION

It is, therefore, a principal object of the present invention to producelow porosity, low dielectric ceramic composites having high strengthproperties.

It is a further object of the present invention to provide a novelmethod for making such ceramic composites.

It is an additional object to provide low dielectric fiber reinforcedcomposites capable of producing large microwavable containers.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a ceramic composite isprovided comprising 35-60 volume percent of a ceramic matrix derivedfrom an amorphous polymer, 20-60 volume percent of a ceramic fiberreinforcement, and having less than 10% porosity. Such compositesexhibit real dielectric constants below 5.0 with loss tangents below0.01, and can withstand temperatures in excess of 2000 degreesfahrenheit. In addition, tensile strengths are in excess of 20,000 psiand interlaminar shear strengths are in excess of 2000 psi. Thecomposites of the present invention are useful in any high temperatureapplication wherein a low dielectric constant is required for bestperformance. They are particularly useful as microwave containers.

The ceramic matrix of the present invention generally comprise polymersformulated from silicon, oxygen, carbon and optionally nitrogen. The lowdielectric properties of the present invention are achieved by producingthe composite in accordance with a method which results in essentiallynone of the carbon being present as free elemental carbon. The amorphouspolymer of the matrix may include, for example, various polysilazane orpolysiloxane polymers and blends thereof. These are available fromHuls-America, Ethyl Corp., Chisso Co. of Japan, and Allied Signal, whichgive ceramic yields in excess of 70%. The polymers must be fiberreinforced and pyrolyzed in inert atmosphere at temperatures below 2000°F. During pyrolysis, the original silicon or polysilazane polymerthermosets to form a network of amorphous ceramic, in which silicon israndomly bonded to either carbon, oxygen, or nitrogen. However, smallamounts of elemental carbon are also formed, which cause high realdielectric constants in the ceramic material and high loss tangents.

Although amorphous ceramic, free of elemental carbon, can be produced bypyrolysis in gaseous ammonia, the use of gaseous ammonia is normallyavoided. A more desirable method of producing low dielectric compositesis to purify the pyrolyzed matrix by low temperature (temperatures aslow as 700° F.-1300° F. can be used, while the upper temperature limitis dictated by thermal stability of other components such as the ceramicfiber), air oxidation after the inert atmosphere pyrolysis, to removetraces of elemental carbon from the matrix. We have found dielectricproperty measurements to be the most sensitive method to monitor theremoval of low levels of carbon, which can have an extremely negativeeffect on dielectric properties. In fact, our dielectric propertymeasurements show very large and unanticipated improvements through postoxidation of the initially produced ceramic composites. Specificconditions for this oxidation will depend upon other components in thesystem. In order to prevent deleterious effects upon components, such asboron nitride fiber coatings, the lowest temperature oxidation possible(i.e., ≧700° F.) is beneficial.

Fibers and particulate fillers are used in the present invention toreinforce the matrix and improve properties. The fibers can becontinuous, discontinuous, or a combination of both types. Furthermore,the fibers' lengths, diameters, and aspect ratios can effect theproperties of the composite material. In particular, if the fibers haveless than a 50:1 aspect ratio, they are ineffective in toughening thematrix by a fiber pullout mechanism. In addition, fiber pullouteffectiveness is increased by coating the fibers with a low modulusmaterial. In the case of these low dielectric composites, a particularlypreferred material for such fiber coating is boron nitride.

Fiber diameter can affect composite strength because smaller diameterfibers are generally stronger than large fibers. However, smallerdiameter fibers, if producibile, can present greater problems inhandling and in alignment. Therefore, the optimum composite strengthdepends on the trade-offs between such factors. Preferably, fibers havea length of at least 500 microns, fiber diameters of between 5 and 15microns, and fiber aspect rations above 50:1. Properties such ascomposite strength, modulus, or density are often linearly dependent offiber volume. The composite may comprise 20-60 volume percent of aceramic fiber reinforcement, preferably 45-55 volume percent.

Alignment of fibers to be parallel to a uniaxial tensile load providesmaximum composite strength. Trade-offs between fiber orientation and thedegree of composite anisotropy desired will depend on application.Various fiber orientations can be used including unidirectional fiberprepreg, 2D cloth weaves, 3D woven structures, and random fiberorientation. Various materials may be used for the ceramic fiberreinforcement, provided the fibers have satisfactory strength anddielectric properties. The ceramic fiber reinforcement should have aroom temperature dielectric constant of less than 6.5 at microwavefrequencies, which frequencies are defined as 2-18 GHz. Examples ofsuitable fibers include boria-mullite fibers such as Nextel 312, 440 and550 (3M), mullite-alumina fibers such as Altex (Sumitomo), highresistivity silicon oxycarbide fiber, such as Nicalon HVR (NipponCarbon), and silicon-carbonitride fiber, such as HPZ (Dow Corning). Alsosuitable are silica fibers such as astroquartz (JPS) for lowertemperature application. Discontinuous fibers such as chopped versionsof the above or mullite fibers, such as Fiberfrax (Carborundum) can alsobe used. In the best mode of operation, these fibers should be coatedwith a low dielectric, low modulus ceramic. Boron nitride coatings,produced by chemical vapor deposition, are preferred.

Low dielectric oxide fillers such as silica-containing glass or ceramiccan also reduce shrinkage during pyrolysis and thus are advantageous.Fillers such as particulate boron nitride, silicon nitride, and mullitecan be advantageous to final properties, and can be applied to thereinforcing fibers during prepregging with liquid polymer or polymersolutions.

To obtain dielectric constants below 5.0 at low porosity, all componentsmust have low real dielectric constants and loss tangents. In general,the rule of mixtures applies to the calculation of the dielectricconstants of these composites. The key to effective utilization ofspecific combinations of fibers, fillers and polymers lies in the properdesign and processing of the composite structure. To this end, there isprovided examples of the method of the present invention, herein.

EXAMPLE 1 Chemical Composition of Pyrolyzed Polymers

To define the composition of ceramic matrices suitable for thisinvention, polymers were pyrolyzed in argon and three determinationswere run: 1) elemental analysis, 2) silicon-29 magic angle spectra NMR,and 3) weight loss studies. Three polymers were studied, a polysilazanepolymer supplied by Chisso Company (NPC 200), a polysiloxane polymersupplied by Allied Signal (Black-glas 489C), and a polysilane polymersupplied by Union Carbide (Y12044). Elemental analyses are shown inTable 1 for polymers pyrolyzed to 1650° F. X-ray diffraction of thesechars does not show crystalline phases so silicon NMR was run todetermine whether oxygen, carbon, or nitrogen is attached to silicon.These spectra, shown in FIG. 1, verify that a statistical distributionof carbon, nitrogen and oxygen is bonded to silicon. This is especiallyevident for the silicon-oxycarbide ceramic where a collection of peaksis seen versus the two peaks that would be expected for a mixture ofsilicon carbide and silica. The presence of elemental carbon isdifficult to quantitate by these methods. However, air oxidation ofceramic chars shows a weight loss that can be correlated by dielectricmeasurements with a carbon content of the chars. This weight loss showsthe removal of elemental carbon by the formation of gaseous carbonoxides. In FIG. 2, we show such weight loss measurements for the charsproduced in this example. It can be seen that simple argon pyrolysisproduces chars that can contain considerable amounts of free carbon.

EXAMPLE 2 Synthesis and Dielectric Properties of BN Composites UsingDifferent Pyrolysis Methods

The dielectric properties of ceramic composites were determined by usingargon pyrolysis and ammonia atmosphere pyrolysis of moldings producedfrom 50 weight percent boron nitride filled polymer moldings producedfrom polysilazane polymer. The dielectric properties of the ceramicproduced by argon pyrolysis are considerably inferior to those producedby pyrolysis in an ammonia atmosphere, as can be seen in FIG. 3.

However, in an additional experiment, the argon pyrolyzed sample wasoxidized in an air environment at 1000° F. As is shown in FIG. 3, thedielectric properties of the ceramic composite are dramatically improvedfrom room temperature to 1500° F. The choice of 1000° F. oxidationtemperature minimizes the oxidation of the boron nitride filler, yeteffectively removes the elemental carbon from the ceramic to the pointwhere dielectric properties are very similar to the composite pyrolyzedin ammonia atmosphere.

EXAMPLE 3 Preparation of Low Dielectric Fiber Reinforced Ceramics byControlled Atmosphere Pyrolysis

A one square meter piece of ceramic fiber cloth (Nicalon HVR-8 HarnessSatin Weave) was treated with a toluene solution of Chisso NCP 200polysilazane polymer and dried to form a prepreg cloth containing 50% byweight of polymer. The cloth prepreg was cut into 16 8 inch by 8 inchsquares and stacked up into 2 8-ply 8×8 inch samples. Both samples werebagged in dammed stacks, wrapped with bleeder cloth and installed in astandard vacuum bag to be autoclave cured. While putting the bag undervacuum, the stacks were slowly cured in an autoclave to 400° F. under100 psi of nitrogen pressure. One of the resulting polymer laminates wasfired in a furnace using an argon atmosphere and a slow pyrolysis cycleto 1650° F. The second laminate was subjected to the same pyrolysiscycle in an ammonia atmosphere. The resulting laminates werere-impregnated with a 50% solution of the polymer in toluene, dried andfired in the same atmosphere in which they were originally pyrolyzed.The reimpregnation-pyrolysis process was repeated ten times to form twoceramic composites with specific gravities of approximately 2.1 g/cc.The argon pyrolyzed composite was then fired in an air atmosphere at900° F. for 18 hours. Both the ammonia fired composite and the airpurified composite were analyzed for dielectric properties by cutting 7mm toroids and testing the toroids in a coaxial stripline cavity at roomtemperature to 1500° F. Results of the tests are shown in FIG. 4. Bothcomposites had real dielectric constants of 4.0 at 10 GHz, with very lowloss tangent.

Comparison Example Preparation of a Fiber Reinforced Ceramic WithoutControlled Atmosphere

Example 3 was repeated to form an 8-ply 8×8 inch ceramic composite thatwas pyrolyzed only in argon and was not subjected to any post pyrolysispurification. This composite was also machined into a 7 mm toroid anddielectric properties tested in a coaxial stripline cavity by the sameprocedures of Example 3. Results of this test are also shown in FIG. 4.This composite had a dielectric constant of 4.9 at 10 GHz and a highloss tangent, especially at frequencies below 10 GHz.

EXAMPLE 4 Production and Dielectric/Mechanical Properties of Filled CMC

Composites were prepared similarly to those in Example 3, usingdifferent combinations of polymers and ceramic fibers. Dielectricconstant data, obtained from coaxial testing is summarized in Table 2.As can be seen, dielectric constant below 5.0 are obtained with allsamples, and high strengths are obtained with BN-coated fibers.

                  TABLE 1                                                         ______________________________________                                        Compositions of Ceramic Chars Produced From Silicon Polymers                                 % Si % C      % N    % O                                       ______________________________________                                        Polysilazane (Si--N--C)                                                                        55     14       28   1                                       Polysiloxane (Si--C--O)                                                                        42     20       --   38                                      Polysilane (Si--C)                                                                             55     42       --   3                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        CMC Properties                                                                Ceramic                                                                              Fiber/Coating                                                                              Fiber (v/o)                                                                             UTS (kis)                                                                            E.sup.1 (10 GHz)                         ______________________________________                                        Si--N--C                                                                             Nicalon HVR/None                                                                           40        <10    4.5                                      Si--O--C                                                                             Nicalon HVR/None                                                                           40        <10    4.4                                      Si--O--C                                                                             Astroguartz/None                                                                           40        <10    3.5                                      Si--N--C                                                                             Nicalon HVR/BN                                                                             40        28     4.8                                      Si--O--C                                                                             Nextel 550/BN                                                                              28        23     4.3                                      Si--O--C                                                                             Altex/BN     30        25     4.9                                      ______________________________________                                    

What is claimed is:
 1. A fiber reinforced ceramic composite with a real dielectric constant of <5.0 comprising 50-60 volume percent of an amorphous polymer matrix, free of elemental carbon and 20-60 volume percent of ceramic fibers coated with a low dielectric coating having a room temperature dielectric constant of less than 6.5, and a loss tangent of <0.01 when measured at 10 GHZ, a porosity of less than 10% and a tensile strength of greater than 20,000 psi.
 2. A fiber reinforced ceramic composite as recited in claim 1 wherein the low dielectric coating is boron nitride. 